Method of generating halftone print data that accommodates overlapping printhead chips

ABSTRACT

A method of generating halftone print data for overlapping end portions of a pair of consecutive printhead chips in an array of two or more printhead chips where an end portion of a first printhead chip overlaps an end portion of a second printhead chip, each printhead chip including a plurality of ink ejection nozzles, includes generating continuous tone print data for the array of printhead chips, The continuous tone print data for said overlapping end portions is interpolated according to an algorithm such that ink ejection nozzle utilization decreases in each said overlapping end portion towards respective ends of the printhead chips.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of U.S. application Ser. No.11/007,319 filed Dec. 9, 2004, which is a Continuation Application ofU.S. application Ser. No. 10/270,153 filed Oct. 15, 2002, now issuedU.S. Pat. No. 6,834,932, which is a Continuation of U.S. applicationSer. No. 09/575,117 filed May 23, 2000, now issued U.S. Pat. No.6,464,332 all of which are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of ink jet printing and inparticular discloses a method and apparatus for the compensation for thetime varying nozzle misalignment of a print head assembly havingoverlapping segments.

CO-PENDING APPLICATIONS

Various methods, systems and apparatus relating to the present inventionare disclosed in the following co-pending applications filed by theapplicant or assignee of the present invention with the presentapplication:

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The disclosures of these co-pending applications are incorporated hereinby cross-reference.

BACKGROUND OF THE INVENTION

In the applicant's co-pending application PCT/AU98/00550, a series ofink jet printing arrangements were proposed for printing at high speedsacross a page width employing novel ink ejection mechanisms. Thedisclosed arrangements utilized a thermal bend actuator built as part ofa monolithic structure.

In such arrangements, it is desirable to form larger arrays of inkejection nozzles so as to provide for a page width drop on demand printhead. Desirably, a very high resolution of droplet size is required. Forexample, common competitive printing systems such as offset printingallow for resolutions of one thousand six hundred dots per inch (1600dpi). Hence, by way of example, for an A4 page print head which is eightinches wide, to print at that resolution would require the equivalent ofaround 12800 ink ejection nozzles for each colour. Assuming a standardfour colour process, this equates to approximately fifty one thousandink ejection nozzles. For a six colour process including the standardfour colours plus a fixative and an IR ink this results in 76800 inkejection nozzles. Unfortunately, it is impractical to make largemonolithic print heads from a contiguous segment of substrate such as asilicon wafer substrate. This is primarily a result of the substantialreduction in yield with increasing size of construction. The problem ofyield is a well studied problem in the semi-conductor industry and themanufacture of ink jet devices often utilizes semi-conductor oranalogous semi-conductor processing techniques. In particular, the fieldis generally known as Micro Electro Mechanical Systems (MEMS). A surveyon the MEMS field is made in the December 1998 IEEE Spectrum article byS Tom Picraux and Paul J McWhorter entitled “The Broad Sweep ofIntegrated Micro Systems”.

One solution to the problem of maintaining high yields is to manufacturea lengthy print head in a number of segments and to abut or overlap thesegments together. Unfortunately, the extremely high pitch of inkejection nozzles required for a print head device means that the spacingbetween adjacent print head segments must be extremely accuratelycontrolled even in the presence of thermal cycling under normaloperational conditions. For example, to provide a resolution of onethousand six hundred dots per inch a nozzle to nozzle separation ofabout sixteen microns is required.

Ambient conditions and the operational environment of a print head mayresult in thermal cycling of the print head in the overlap regionresulting in expansion and contraction of the overlap between adjacentprint head segments which may in turn lead to the production ofartifacts in the resultant output image. For example, the temperature ofthe print head may rise 25° C. above ambient when in operation. Theassembly of the print head may also be made of materials havingdifferent thermal characteristics to the print head segments resultingin a differential thermal expansion between these components. Thesilicon substrate may be packaged in elastomer for which the respectivethermal expansion coefficients are 2.6×10⁻⁶ and 20×10⁻⁶ microns perdegree Celsius.

Artifacts are produced due to the limited resolution of the print headto represent a continuous tone image in a binary form and the ability ofthe human eye to detect 0.5% differences in colour of adjacent dots inan image.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide for a mechanism forcompensating for relative displacement of overlapping print headsegments during operation in an effective and convenient manner.

In accordance with a first aspect of the invention there is provided inan ink ejection print head comprising a plurality of overlapping printhead segments, wherein the spatial relationship between adjacentsegments is variable with time, a method for controlling the firing ofnozzles within the overlapped segments comprising the steps of: (a)determining a measure of the overlap between adjacent print headsegments; (b) creating a half toning pattern for the nozzles in theregion of overlap of the overlapping segments; and (c) adjusting saidhalf toning pattern as a function of said measure in the overlappingregions of said print head segments to reduce artifacts produced by theoverlapping of said print head segments.

Preferably, the step for determining a measure of overlap employs ameasure of temperature of the print head segments The half toningpatterns are preferably produced by means of a dither matrix or dithervolume and the alteration can comprise adding an overlap value to acurrent continuous tone pixel output value before utilizing the dithermatrix or dither volume. In place of a measure of temperature a measureof distance can be provided by the use of fiduciary strips on each ofthe segments and using an interferometric technique to determine thedegree of relative movement between the segments.

In accordance with a further aspect of the present invention, there isprovided an ink ejection print head system comprising: a plurality ofspaced apart spatially overlapping print head segments; at least onemeans for measurement of the degree of overlap between adjacent printhead segments; means for providing a half toning of a continuous toneimage and means for adjusting said half toning means in a region ofoverlap between adjacent print head segments to reduce artifacts betweensaid adjacent segments.

The means for adjusting the half toning means can include a continuoustone input, a spatial overlap input and a binary input, the half toningmeans utilizing the spatial overlap input to vary the continuous toneinput to produce a varied continuous tone input for utilization in alook-up table of a dither matrix or dither volume so as to produceoutput binary values to adjust for the regions of overlap of print headsegments. The means for adjusting the half tone or dither matrix may beimplemented in hardware or by means of software employing an algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 shows a schematic of a pair of adjacent print head segmentsaccording to the invention;

FIG. 2 illustrates the process for printing dots from adjacent printhead segments as shown in FIG. 1;

FIG. 3 illustrates a process of blending dots between adjacent printhead segments according to the invention;

FIG. 4 illustrates a process of dither matrix variational controlaccording to an embodiment of the invention;

FIG. 5 illustrates a process of dither matrix variational controlaccording to another embodiment of the invention; and

FIG. 6 illustrates graphically an algorithm implementing a furtherprocess of dither matrix variational control according to a furtherembodiment of the invention.

FIG. 7 shows a schematic of a pair of adjacent printhead segmentsaccording to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In a first embodiment, a method of compensation for the temperaturevarying relative displacement of adjacent print head segments isprovided by the utilization of a digital processing mechanism whichadjusts for the overlap between adjacent segments.

In a print head covering an A4 page width there may be 10 segmentshaving 9 overlapping portions arranged in a repeating sequence ofstaggered pairs. Initial alignment of segments can be made within 10microns using techniques well known in the art of monolithic fabricationtechniques. The width of a segment for a 6 colour ink arrangement wouldbe approximately 225 microns assuming the nozzles of a segment arearranged on 16 micron centres in a zig-zag pattern longitudinally.

In this embodiment, a temperature sensor is placed on each print headsegment so as to provide for a measure of the current temperaturecharacteristics of each print head segment. The current temperaturemeasurement can then be utilized to determine the amount of overlapbetween adjacent print head segments.

Alternatively, only a single temperature sensor can be used if it can beassumed that the segments of the print head are sufficiently similar toone another in physical characteristics and performance and that theambient milieu of each pair of overlapped segment is substantially thesame.

The degree of overlap is then used to provide a mechanism forcontrolling the half toning between adjacent print head segments. It isassumed that outputting of an image in the instant invention is by meansof digital half toning employing any method or technique well known inthe art. Many different half toning techniques can be utilized andreference is made to the text by Ulichney entitled “Digital Half Toning”published by MIT Press.

As shown in FIG. 1 adjacent print head segments 2, 3 overlap in therespective regions 12, 13. The overlap region may extend approximately40 thou (˜1 mm.) providing an overlap of 64 nozzles spaced at 16 micronsfor 1600 dpi resolution.

A temperature sensor 16 is placed on each print head segment 2, 3 so asto provide for a measure of the current temperature characteristics ofeach print head segment 2, 3. The current temperature measurement canthen be utilized to determine the amount of overlap between adjacentprint head segments. Alternatively, fiduciary strips 100, 101 on eachoverlapped segment 102, 103, as shown in FIG. 7, may be used to measurethe degree of relative displacement of the segments 102, 103 by aninterferometric technique.

In the region 10 of the segment 2 the nozzles of this segment are usedexclusively for the ejection of ink. Similarly in the region 11 of thesegment 3 the nozzles of this segment are used exclusively for theejection of ink. In the overlapping regions 12, 13 a “blend” is providedbetween the two print head segments 2, 3 such that along the edge 14 ofthe print head segment 2 nozzles are used exclusively in the region 12to print and similarly along the edge 15, the nozzles of the segment 3are used almost exclusively for printing. In between, an interpolation,which can be linear or otherwise, is provided between these two extremepositions. Hence, as shown in FIG. 2, when printing a full colour outputon a page the area on the side 17 is printed exclusively by the printhead segment 10 while the area 18 is printed exclusively by the printhead segment 11 (as illustrated by the black dots) with the area 19comprising a blend between the nozzles of the two segments. The printingprocess utilizes any well known half toning matrix such as disclosed inthe aforementioned references. While a known half toning matrix isutilized, the actual print head segment utilized will depend upon theblending ratio provided by the measure of overlap between theoverlapping segments.

One such method is illustrated in FIG. 3 where a linear interpolationwithin the overlapped regions is shown. In the region corresponding tothe overlapped section 12 at the edge 14 there is 100% utilization ofthe nozzles of print head segment 2, whereas in the equivalent region,edge 7, of the print head segment 3 there is zero output. As thedistance of the overlap region from the line 14 of the segment 2 isincreased towards the line 15 of the segment 3 the proportion ofutilization of the nozzles of the section 12 is gradually decreased(linearly), being zero at edge 9 while the utilization of the nozzles ofthe section 13 is progressively increased to unity by the time the edge15 is reached. In a first embodiment, where there is an increasedoverlap between nozzles, the half toning thresholds utilized areincreased in the overlap region. This reduces the number of dots printedin the blend region. Conversely, if there is a reduced overlap with theprint head segments being spaced apart slightly more than normallyacceptable, the dot frequency can be increased by reducing the halftoning threshold.

An overall general half toning arrangement can be provided as shown inFIG. 4 with a dither matrix 25 outputting a current dither value 26 to asummation means 27 with summation means 27 having another input 28, anoverlap signal, which varies in either a positive or a negative sensedepending on the degree of overlap between the adjacent segments. Theoutput value 29 of summation means or adder 27 is compared to the inputcontinuous tone data 32 via a comparator 30 so as to output half tonedata 31. An alternative arrangement allows that the data value 28 can besubtracted from the continuous tone data 29 before dithering is appliedproducing similar results. This arrangement is shown in FIG. 5.

As shown in FIG. 5, a halftone data output 52 can be generated bycombining the output 42 of dither matrix 40 in an adder 46 with theoverlap signal 44, and then taking the difference of the output 54 ofadder 46 and the continuous tone data 48 in subtracter 50. This is anequivalent arrangement to that of FIG. 4.

Through the utilization of an arrangement such as described above withrespect to FIGS. 3 and 4, a degree of control of the overlap blendingcan be provided so as to reduce the production of streak artifactsbetween adjacent print head segments.

As each overlap signal 28 can be multiplied by a calibration factor andadded to a calibration offset factor, the degree of accuracy ofplacement of adjacent print head segments can also be dramaticallyreduced. Hence, adjacent print head segments can be roughly alignedduring manufacture with one another. Test patterns can then be printedout at known temperatures to determine the degree of overlap betweennozzles of adjacent segments. Once a degree of overlap has beendetermined for a particular temperature range a series of correspondingvalues can be written to a programmable ROM storage device so as toprovide full offset values on demand which are individually factored tothe print head segment overlap.

A further embodiment of the invention involves the use of a softwaresolution for reducing the production of artifacts between overlappedsegments of the print heads. A full software implementation of a dithermatrix including the implementation of an algorithm for adjustingvariable overlap between print head segments is attached as appendix A.The program is written in the programming language C. The algorithm maybe written in some other code mutatis mutandis within the knowledge of aperson skilled in the art. The basis of the algorithm is explained asfollows.

A dispersed dot stochastic dithering is used to reproduce the continuoustone pixel values using bi-level dots. Dispersed dot ditheringreproduces high spatial frequency, that is, image detail, almost to thelimits of the dot resolution, while simultaneously reproducing lowerspatial frequencies to their full intensity depth when spatiallyintegrated by the eye. A stochastic dither matrix is designed to be freeof objectionable low frequency patterns when tiled across the page.

Dot overlap can be modelled using dot gain techniques. Dot gain refersto any increase from the ideal intensity of a pattern of dots to theactual intensity produced when the pattern is printed. In ink jetprinting, dot gain is caused mainly by ink bleed. Bleed is itself afunction of the characteristics of the ink and the printing medium.Pigmented inks can bleed on the surface but do not diffuse far insidethe medium. Dye based inks can diffuse along cellulose fibres inside themedium. Surface coatings can be used to reduce bleed.

Because the effect of dot overlap is sensitive to the distribution ofthe dots in the same way that dot gain is, it is useful to model theideal dot as perfectly tiling the page with no overlap. While an actualink jet dot is approximately round and overlaps its neighbours, theideal dot can be modelled by a square. The ideal and actual dot shapesthus become dot gain parameters.

Dot gain is an edge effect, that is it is an effect which manifestsitself along edges between printed dots and adjacent unprinted areas.Dot gain is proportional to the ratio between the edge links of a dotpattern and the area of the dot pattern. Two techniques for dealing withdot gain are dispersed dot dithering and clustered dot dithering. Indispersed dot dithering the dot is distributed uniformly over an area,for example for a dot of 50% intensity a chequer board pattern is used.In clustered dot dithering the dot is represented with a single central“coloured” area and an “uncoloured” border with the ratio of the area of“coloured” to “uncoloured” equalling the intensity of the dot to beprinted. Dispersed dot dithering is therefore more sensitive to dot gainthan clustered dot dithering.

Two adjacent print head segments have a number of overlapping nozzles.In general, there will not be perfect registration between correspondingnozzles in adjacent segments. At a local level there can be amisregistration of plus or minus half the nozzle spacing, that is plusor minus about 8 microns at 1600 dpi. At a higher level, the number ofoverlapping nozzles can actually vary.

The first approach to smoothly blending the output across the overlapbridge and from one segment to the next consists of blending thecontinuous tone input to the two segments from one to the other acrossthe overlap region. As output proceeds across the overlap region, thesecond segment receives an increasing proportion of the input continuoustone value and the first segment receives a correspondingly decreasingproportion as described above with respect to FIG. 3. A linear or higherorder interpolation can be used. The dither matrices used to dither theoutput through the two segments are then registered at the nozzle level.

The first approach has two drawbacks. Firstly, if the dither thresholdat a particular dot location is lower than both segments' interpolatedcontinuous tone values then both segments will produce a dot for thatlocation. Since the two dots will overlap, the intensities promised bythe two dither matrices will be only partially reproduced, leading to aloss of overall intensity. This can be remedied by ensuring thatcorresponding nozzles never both produce a dot. This can also beachieved by using the inverse of the dither matrix for alternatingsegments, or dithering the continuous tone value through a single dithermatrix and then assigning the output dot to one or the other nozzlestochastically, according to a probability given by the currentinterpolation factor.

Secondly, adjacent dots printed by different segments will overlap againleading to a loss of overall intensity.

As shown in FIG. 6, the value for each overlapped segment is plottedalong the horizontal axes 60, 62 as V_(A) and V_(B) respectively betweenthe values of 0.0 and 1.0. The calculated output 66 is plotted withrespect to the vertical axis 64 as a function, I_(A+B), for valuesranging from 0.0 to 1.0. A contour plane 68 shows the resultant valuesfor I_(A+B)=0.5.

FIG. 6 shows the qualitative shape of the three dimensional functionlinking the two segments' input continuous tone values V_(A) and V_(B)to the observed output intensity I_(A+B). For the first approach, aninput continuous tone value V and an interpolation factor f togetheryield V_(A)=(1−f) V and V_(B)=fV. The closer the interpolation factor isto 0.5 the greater the difference between the input continuous tonevalue and the observed output intensity. For V=1.0, this is illustratedin FIG. 6 by the curve 200 on the vertical V_(A)+V_(B)=1.0 plane. Bydefinition this curve lies on the function surface. FIG. 6 indicatesthat when any kind of mixing occurs, that is 0.0<f<1.0, the outputintensity is attenuated, and to achieve the desired output intensity thesum of the two segments' input values must exceed the desired outputvalue, that is V_(A)+V_(B)>V. This forms the basis for the algorithm inappendix A.

The function shows a linear response when only one segment contributesto the output, that is f=0.0 or f=1.0. This assumes of course that thedither matrix includes the effects of dot gain.

The foregoing description has been limited to specific embodiments ofthis invention. It will be apparent, however, that variations andmodifications may be made to the invention, with the attainment of someor all of the advantages of the invention. For example, it will beappreciated that the invention may be embodied in either hardware orsoftware in a suitably programmed digital data processing system, bothof which are readily accomplished by those of ordinary skill in therespective arts. Therefore, it is the object of the appended claims tocover all such variations and modifications as come within the truespirit and scope of the invention.

Appendix A

A full software implementation of a dither matrix including theimplementation of an algorithm for adjusting variable overlap betweenprint head segments is provided below. The program is written in theprogramming language C.

1. A method of generating halftone print data for overlapping endportions of a pair of consecutive printhead segments in an array of twoor more printhead segments where an end portion of a first printheadsegment overlaps an end portion of a second printhead segment, eachprinthead segment including a plurality of ink ejection nozzles, themethod comprising the steps of: generating continuous tone print datafor the array of printhead segments; generating an initial dither matrixto be applied to said continuous tone print data; generating an overlapsignal representing an extent of overlap of the end portions; and,interpolating the continuous tone print data for said overlapping endportions according to an algorithm such that ink ejection nozzleutilization decreases in each said overlapping end portion towardsrespective ends of the printhead segments.
 2. A method as claimed inclaim 1, which includes the step of generating an initial dither matrixto be applied to said continuous tone print data and the step ofgenerating an overlap signal representing an extent of overlap of theend portions.
 3. A method as claimed in claim 2, in which the step ofapplying the overlap data to the initial dither matrix includes the stepof receiving, as an input to an adder, a current dither valuerepresenting the dither matrix and the overlap signal.
 4. A method asclaimed in claim 2, in which the step of applying the output value tothe continuous tone print data includes the step of receiving, as aninput to a comparator, a signal representing the output value and asignal representing the continuous tone print data.
 5. A method asclaimed in claim 2, in which the step of applying the output value tothe continuous tone print data includes the step of receiving, as aninput to a subtractor, a signal representing the output value and asignal representing the continuous tone print data.
 6. A method asclaimed in claim 1, in which the step of generating the overlap signalincludes the step of measuring a temperature of the printhead segmentsto determine an extent of thermal expansion or contraction of theprinthead segment.
 7. A method as claimed in claim 1, in which the stepof generating the overlap signal includes the step of determiningrelative displacement of the printhead segments using fiducial stripspositioned on the printhead segments and an interferometric technique.